1. Field of the Invention
The present invention relates to digital communication between two stations; and more particularly to a system and method for frequency acquisition in digital communication systems.
Although the present invention is suitable for use in many types of digital communication systems, it is particularly advantageous in cellular communication systems for determining the frequency offset between a cellsite and the local oscillator of a mobile receiver; and is described in connection therewith.
2. Discussion of Related Art
A cellular communication system is a mobile telephone service wherein radio coverage is divided into cells, each of which may cover an area in the neighborhood of one to two square miles. Each cell is assigned a number of available radio frequencies. The same frequencies or channels used in one area or cell are also used for areas that are spatially separated from one another. A mobile telephone station transmits and receives control and voice communication information to and from a base station, commonly referred to as a cellsite, located within the same cell. The base stations are controlled by a cellular system switching and control network that provides connection with the worldwide telecommunications network.
A call in progress is not interrupted as the mobile station travels from one cell location to another, since the system provides for automatic reassignment to an available time slot of an available channel within the other cell commonly referred to as a handoff.
In order to provide superior non-interfering communication and compatibility among many different base stations and mobile stations in different parts of the world, various operational and material specifications and standards were developed, which all suppliers and users are obliged to follow. For example, a mobile station that operates in the digital mode is required to use an RF band which is divided into two separate twenty-five Mhz wide segments, each consisting of eight hundred thirty-two channels. The first segment contains the mobile station transmit channels, and the second segment contains the mobile station receive channels. Thus, each transmit and receive channel is approximately thirty kilohertz or kilocycles in width.
Each channel has a frame format; that is, each channel radiates a succession of frames, each of which has a duration of forty milliseconds, and constitutes one cycle of a regularly recurring series. Each frame has six time slots, and each slot has one hundred sixty-two data symbols and a duration of 6.67 milliseconds, for example.
The term frequency offset as used herein is the difference between the frequency of the local oscillator in the mobile receiver and the frequency transmitted by the base station of the cell. In order for a demodulator in a digital cellular phone to successfully acquire a signal from a base station, the frequency offset should be less than eight hundred Hz, in that synchronization of the frame requires that the frequency offset be within eight hundred Hz in order for the bit timing to be correct. Any greater offset will result in the call being dropped if there is a transfer to another cell. This frequency offset should be reduced to within plus or minus two hundred Hz(minus the transmit/receive difference of forty-five Mhz). This two hundred Hz offset is also the maximum initial frequency offset which the demodulator automatic frequency control (AFC)loop is able to tolerate.
A frequency offset between a base station and a mobile receiver can occur for several reasons, such as difference in ambient temperature, aging of the components over a period of time, and assignment of a channel by a new base station, for example. Also, in digital to digital communication where two cellsites are at the limit of their tolerance, a handoff from one to the other can appear to be offset by as much as 610 Hz. During analog to digital handoffs, the initial frequency offset may be much larger. For example, the initial frequency in the worst case may be offset by 4500 Hz because of the tolerance of the voltage controlled oscillator.
Therefore, in order to meet the required maximum frequency offset of .+-.200 Hz, it is necessary that the frequency of the mobile station can be varied over a certain range to minimize any frequency offset. The carrier frequency of the receiver is determined by a voltage controlled local oscillator (VCXO) that is tunable over a certain range, such as forty-five hundred Hz. Prior to the present invention, various systems were provided for controlling the voltage of a VCXO to vary the frequency of the radio receiver. Typically, samples of the received waveform were obtained and correlated, with the VCXO being tuned in accordance with the results. If, after tuning, the frequency offset was still excessive, more samples would be taken, and the control voltage of the VCXO again would be varied by a certain amount. This process would be repeated until the offset frequency came within the desired limits.
In U.S. Pat. No. 4,644,561, a frequency acquisition routine is described that takes advantage of the period of time during which there is no transmission from the base station. At the expiration of this time the base station transmits an unmodulated carrier signal, which causes the IF mixer of the receiver to output another sine waveform whose frequency is proportional to the difference between the VCXO and the base station frequency. The modem program samples the I,Q channels at certain intervals and determines the phase change for each interval, puts it through a low pass filter and sends it as a correction word to control the VCXO. Frequency acquisition is achieved when the phase change becomes lower than a certain level.
Although suitable for the purposes intended, the methods and systems for frequency acquisition prior to the present invention tended to require costly precision voltage controlled oscillators, and the receipt of multiple frames of data in order to effect proper frequency acquisition.